Luk 21:29 And he gave them an illustration: “Look at the fig tree, and all the trees.
Luk 21:30 As soon as they come out in leaf, you see for yourselves and know that the summer is already near.
Luk 21:31 In the same way, when you see these things taking place, you know that the kingdom of God is near.
Luk 21:32 I am telling you the truth; this generation will not go by until all has begun to take place.
Luk 21:33 The sky and the land will go by, but my words will not go by.

no matter what goes by

Jesus had begun his eschatological discourse responding to a request for signs. Jesus shared a number of signs, some signifying the soon destruction of Jerusalem, others indicating that the end of the age and the second coming is near. In this illustration, he warns his listeners (and readers) not to ignore the signs. All of the signs will have begun to take place before his generation goes by.

But Jesus also warns us (in the same illustration) that the signs of the end will take place over a long period of time, and people will be tempted to think that his words have gone by with no affect. No matter how long it takes, even if it seems the sky and land itself has died of old age, his words are true. His return is going to happen, and the signs he gave are the only indicators that the age is ending.